The present invention relates to the field of diffraction gratings, and more specifically to switchable holographic gratings formed from materials comprising polymer-dispersed liquid crystal cells.
Diffraction gratings are widely used in optics and optical instruments to separate light of different wavelengths with high resolution. Diffraction gratings create monochromatic light from a source of white light, which is realized because of the grating's ability to spread light of different wavelengths into different angles. Through application of an electric field that modifies the characteristics of the grating, diffraction gratings may be switchable.
Holographic gratings have generally been formed by an interference fringe field of two laser beams whose standing wave pattern is exposed to a polished substrate coated with a photo resist. Processing of the exposed medium results in a pattern of straight lines with a sinusoidal cross section. Because holographic gratings produce less stray light than ruled gratings, they are often preferred. Holographic gratings can also be produced with up to 3600 grooves per millimeter for greater theoretical resolving power.
Polymer-dispersed liquid crystals (PDLCs) are composite materials consisting of a dispersion of sub-micron size droplets of nematic liquid crystal (LC) in a polymer matrix. They are well-known to exhibit switchable optical properties; optical transmission through the PDLCs can be varied between almost zero and 100% by applying electric fields. Recently it has been demonstrated that the optical properties of the PDLCs can be altered significantly by exposure to nuclear gamma radiation as well as, ultraviolet radiation (the Electrochromic efffect). The holographic polymer-dispersed liquid crystal (H-PDLC) cell provides a unique combination of a suitable material and physical mechanism, whereby a permanent hologram can be imprinted and its diffraction efficiency modulated by an applied electric field.
Currently, holographic gratings are written by using relatively expensive and high energy light sources, e.g. green light from an argon-ion laser or UV sources. As such, there remains a need to fabricate holographic gratings more economically and with more efficiency to enable its use for a broader set of applications.